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Only crabs that received shocks vacated their shells during the experiment, indicating that what they had experienced was unpleasant.

With less powerful shocks, below the threshold that forced them out, crabs remained in their shells but appeared to be waiting for an opportunity to move.

When a new shell was offered to them, they were more likely to switch homes than crabs that had not received a shock.

This suggested that the crabs retained a memory of the pain they had felt earlier, said the scientists.

Professor Bob Elwood, from Queen's University, Belfast, who has previously studied evidence of pain in prawns, said: "There has been a long debate about whether crustaceans including crabs, prawns and lobsters feel pain.

"We know from previous research that they can detect harmful stimuli and withdraw from the source of the stimuli but that could be a simple reflex without the inner 'feeling' of unpleasantness that we associate with pain.

"This research demonstrates that it is not a simple reflex but that crabs trade-off their need for a quality shell with the need to avoid the harmful stimulus.

"Such trade-offs are seen in vertebrates in which the response to pain is controlled with respect to other requirements.

"Humans, for example, may hold onto a hot plate that contains food whereas they may drop an empty plate, showing that we take into account differing motivational requirements when responding to pain.

"Trade-offs of this type have not been previously demonstrated in crustaceans. The results are consistent with the idea of pain being experienced by these animals."

In his previous work, Prof Elwood found that when a prawn's antenna was treated with weak acetic acid, the creature rubbed it as if it was sore. But the rubbing was reduced by local anaesthetic.

The findings are consistent with observations of pain in mammals, said the professor, whose latest research is published in the journal Animal Behaviour.

He added: "More research is needed in this area where a potentially very large problem is being ignored.

"Legislation to protect crustaceans has been proposed but it is likely to cover only scientific research.

"Millions of crustacean are caught or reared in aquaculture for the food industry.

"There is no protection for these animals (with the possible exception of certain states in Australia) as the presumption is that they cannot experience pain.

"With vertebrates we are asked to err on the side of caution and I believe this is the approach to take with these crustaceans."